The interiors of can bodies must be coated to protect against corrosion and to insure quality control of the product. For efficient processing of millions of such cans, it is imperative that the interior coating be applied rapidly, uniformly, and economically. To this end, indexing turrets have been employed wherein a coating spray is applied to can interiors as the open-ended cans are indexed past coating-spray guns located at one or more spray stations along the turret indexing route.
A common problem associated with current turret arrangements is the lengthy turret dwell time required for satisfactorily spraying the interior of each can.
A further problem is the misalignment both of the can center with respect to the center of the can moving means and of the can interior with respect to the spray gun. Possible adverse consequences of such misalignment include uneven spray application, spray buildup on some interior surfaces, waste of spray and denting and damaging of the cans.
Inventions of earlier vintage sought to reduce the time spent at spray stations. Accordingly, rather than move nozzles around to spray all portions of a can's interior, cans were rotated to spread the spray around the can interior. Can rotation has generally been imparted by drive belts positioned tangentially to the can and in direct contact with the can exterior. However, direct contact with the drive belts has had a tendency to force the can from the center of its transport means, resulting in the undesirable misalignment discussed above. Further, such direct contact has had a tendency to scratch, dent or otherwise damage the can exterior. Scratches cannot be tolerated, especially if the can has been previously printed with exterior decorations. Moreover, rotation of the can in most earlier devices has been inefficient, since a delay has occurred while the can was being accelerated to a satisfactory rotational velocity at the spray station.
In general, as cans journey through a turret mechanism they are moved by a starwheel structure having either pockets or rollers for the cans; and, when a can is not securely centered on its transport means as it is carried through the turret arrangement, there is the hazard of denting and damaging the cans, which are made as light weight as possible.
Another prior art problem relates to overspray. That is, since manufacturers have had no assurance of consistent, accurate can alignment, they have had to increase the spray dosage to maintain a satisfactory coating on the can interiors. At times, therefore, this excess spray builds up unpredictably and uneconomically in the can interior, and some excess spray must be drawn away through a vent stack and wasted.
The prior art has endeavored to combat the above problems by firmly mounting the can at the spray stations by use of a vacuum means. However, the vacuum mounts have occurred only at the spray station with little or no assurance of accurate can centering with respect to the transport mechanism or the spray gun.
In view of the above problems, an object of this invention is to provide a can spraying apparatus which will economically and uniformly distribute a coating on can interiors.
Another object of this invention is to increase the turret indexing frequency by eliminating the time heretofore wasted by a rotational "warm up" at or proximate the spray station.
A further object of this invention is the reduction of can damage and can dents that have resulted from prior can spraying devices.
Still another object of the invention is to reduce the amount of spray required for coating can interiors; and, even another object of the invention is the reduction of waste by reducing the amount of overspray.